The present invention relates to a proximity image intensifier for use in a light amplifier in a high-sensitivity hand-held camera for broadcasting service or a device for providing night vision.
As shown in FIGS. 1A and 1B, a conventional proximity image intensifier includes a photocathode 10 and a phosphor screen 12 which are disposed closely to each other in a vacuum. A high voltage of 9 kV, for example, is applied from a high-voltage power supply 14 between the photocathode 10 and the phosphor screen 12 through a high resistor 16, and flanges 18, 20, 22, 24 to accelerate the velocity of the photoelectrons emerging from the photocathode 10 dependent upon the incident of optical image thereon. Under the applied high voltage, an optical image entered into the photocathode 10 is intensified and reproduced on the phosphor screen 12. The resistor 16 has a high resistance ranging from 1 G.OMEGA. to 30 G.OMEGA.. The resistor 16 is provided to limit the undue flow of current between the photocathode 10 and the phosphor screen 12 which may occur in an accidental dielectric breakdown therebetween. The resistor 16 further serves to suppress a flow of photoelectrons which are produced when highly intensive light such as flash light falls on the photocathode 10, so that the photocathode 10 and the phosphor screen 12 are prevented from being damaged.
The high resistor 16 in the conventional image intensifier shown in FIGS. 1A and 1B is capable of blocking a photoelectron beam for the protection of the photocathode 10 and the phosphor screen 12 from burnout when highly intensive light such as flash light falls widely over the photocathode 10. However, when intensive incident light is applied only to a small area (e.g., a spot which is 1 mm across) on the photocathode 10, the entire flow of generated photoelectrons is not so large though a localized density of photoelectrons is increased. Therefore, the high resistor 10 is not effective for such an instance, causing to locally burn out the phosphor screen 12.
Research has been conducted to determine possible causes of such a burnout on the phosphor surface 12. Heretofore, the outside diameter of the photocathode 10 is substantially equal to the inside diameter of the flange 18, and the photocathode 10 and the flange 18 are coupled to each other by an electrically conductive layer 21. Consequently, a large substantial electrostatic capacitance C is developed between the photocathode 10 and the phosphor screen 12. It has been found that the electric charge Q (=CV) stored by the electrostatic capacitor C is one of the causes of the burnout. The electrostatic capacitance C is composed of not only the electrostatic capacitance between the photocathode 10 and the phosphor screen 12, but also the electrostatic capacitance between the flanges 18, 20 and 22, 24. Since the size of the photocathode 10 is much larger than the area of an effective portion 10a thereof, the electrostatic capacitance C has a large value of 8 pF, for example.